In Kayser U.S. Pat. No. 1,939,903 and Becquemont U.S. Pat. No., 2,091,953, neon tubes are driven in such a way that the luminous discharge has the appearance of brilliant luminous balls or bubbles separated by dark spaces and appearing to move in the interior of the tube from one electrode to the other. In Kayser, an oscillating current is applied to each end of the neon tube with one of the tube ends being grounded and the direction of travel of the luminous span issuing from the non-grounded end of the tube is varied by varying the characteristics of the oscillating current. In the Kayser patent, a high frequency current has the voltage level thereof varied in a slow and progressive manner according to the apparent movement and development of the bubbles and then the high frequency current is shut-off and a low frequency industrial current is applied in a slow and progressive manner so that the main voltage of the oscillating current is varied. In Becquemont, a high frequency alternating current is applied to the electrodes at a sufficient frequency to produce the luminous bubbles separated by dark regions and a portion of the alternating current is rectified and superimposed on the applied alternating current. In yet another prior art system, a frequency of between 1500 and 4000 Hz is produced and the symmetry of the current applied to the tube is adjusted either by adjusting the clipping one of the peaks of a driving sinusoidal wave or by producing a substantially square wave voltage and adjusting the duty cycle thereof. Such a system is disclosed in Kennan C. Herrick PCT/U.S. Pat. No. 86/00851, International Publication No. W086/06572, incorporated herein by reference application entitled "Apparatus and Method for Forming Segmented Luminosity in Gas Discharge Tubes".
The Herrick PCT application discloses regulation of the symmetry of the resultant tube current, i.e., regulation of the net current flow between the electrodes. The resultant or net current flow through the tube is regulated so as to control the movement of these alternating light and dark regions lengthwise along the tube. Regulation of the net current flow may be accomplished either by imposition of a DC current across the electrodes in addition to the AC signal, or by use of an AC signal having an asymmetric wave form. Asymmetry of the wave form may be measured by determining the DC voltage across a capacitor connected in series with the gas discharge tube, and this measurement may be applied in a feedback loop to control the degree of asymmetry and hence control the rate of movement of the illuminated regions lengthwise along the tube. In a further variant, the tube may be provided with a midpoint electrode to provide net current flows either from the end electrodes towards the midpoint or from the midpoint towards the end, with corresponding movement of the light and dark regions towards or away from the midpoint.